Lithium ion secondary batteries using organic liquid electrolytes have been widely used in small electronic devices due to their excellent characteristics in terms of energy and power densities over other energy storage units.
In recent years, the application of lithium ion secondary batteries using organic liquid electrolytes has been rapidly extended to medium and large energy storage devices as well as small electronic devices. However, leakage of liquid electrolytes increases the risk of explosion or fire. Attention has thus focused on safe electrolyte materials free from any risk of explosion or fire. Under these circumstances, considerable research has been conducted on all-solid-state lithium secondary batteries using nonflammable inorganic solid electrolytes. Oxide, halide, and sulfide solid electrolytes are the most actively investigated inorganic solid electrolytes for all-solid-state lithium secondary batteries. Particularly, sulfide solid electrolytes have received attention as the most promising materials because of their superior lithium ion conductivity.
However, when a sulfide solid electrolyte comes into contact with an oxide active material that is widely in use, undesirable reactions may occur at the interface between the electrolyte and the active material. For example, a resistive layer may be formed by diffusion of the metal elements or a lithium depletion layer may be formed due to different potentials. Such interfacial reactions cause markedly increased interfacial resistance, leading to significant deterioration of cycle characteristics and high-rate characteristics.
Many attempts have been made to suppress interfacial side reactions between sulfide solid electrolytes and oxide active materials and to achieve improved cycle characteristics and high-rate characteristics. A recent report has shown that the formation of a coating layer composed of a transition metal oxide, such as Al2O3, ZrO or SiO2, a lithium transition metal oxide, such as Li4Ti5O12 or LiNbO3, an oxide, such as Li2O—SiO2 or a transition metal sulfide, such as NiS or CoS, on the surface of an oxide active material represented by Li1+x(M)O2 (where M includes at least one transition metal selected from Co, Mn, and Ni and x is from 0 to 1) can suppress side reactions, such as the formation of a resistive layer by diffusion of the metal elements or the formation of a lithium depletion layer due to different potentials.
However, active material coating techniques for suppressing interfacial reactions suffer from limitations in that additional coating processes using starting materials containing metal elements incurs high costs, compositions of coating materials are difficult to precisely control, and optimization of the diffusion coefficient of lithium ions in coating materials requires complex coating process conditions. Such variables greatly limit the choice of suitable coating materials. Thus, there is a need to develop coating materials more suitable for use in all-solid-state batteries and techniques for preparing the coating materials.